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In order to improve power density, the majority of diesel
engines have intake manifold pressures above atmospheric
conditions. This allows for the introduction of more fuel, which
results in more power. Except for a few applications, these engines
receive charged air from a turbocharger. The turbocharger develops
boost by converting exhaust gas energy into power. This power is
then used to compress the intake charge.

The medium- and heavy-duty engine markets have both stringent
regulatory targets and customer demand for improved fuel
efficiency. Two approaches used to meet fuel efficiency targets are
downspeeding and downsizing. Until now, the industry has adapted to
the turbocharger lag experienced during a transient acceleration
event. This performance deficiency is severely exaggerated when the
displacement and speed of an engine are reduced. The solution
proposed to improving fuel economy, while maintaining equivalent
performance, is supercharging.

Eaton's TVS® supercharger is a positive displacement pump
with significantly improved efficiencies over prior models. Unlike
a turbocharger, which relies on exhaust gas energy to create boost,
a supercharger is directly coupled to the engine. Therefore, the
supercharger speed increases proportionally with engine speed. This
greatly reduces the lag that is present in a turbocharged engine.
Addition of a supercharger, as this paper will show, dramatically
improves low speed engine performance. This enables greater engine
downspeeding that was previously not feasible with a turbocharged
engine.

This paper explores the fuel economy benefits of combining a
supercharger with a turbocharger on a diesel engine. The fuel
economy and performance improvements enabled by supercharging are
dependent on application, drive cycle, and engine displacement. It
also reviews the simulation results of three distinct engine sizes
and applications exercised over vehicle appropriate drive cycles.
The effect of boosting system configuration, downspeeding, exhaust
gas recirculation (EGR) configuration, and supercharger clutching
on fuel economy were examined. All of the simulations provided a
fuel economy benefit over the baseline turbocharger-only engines.
Fuel economy was improved, while maintaining or improving vehicle
performance over the baseline. Efficiency improvements in excess of
14% were demonstrated over a transient drive cycle.